The present invention relates to Microelectromechanical Systems (MEMS). More particularly, the present invention pertains to frequency selective MEMS devices.
Currently there is an interest in increasing the degree of integration in electronics. One reason to increase the degree of integration is to construct a system-on-a-chip. In a system-on-a-chip all the electronics for a system, including, for example, sensors, signal processing, and communication circuits are fabricated on a single semiconductor die. Aside from systems-on-a-chip, increasing the degree of integration can lower manufacturing costs, and allow for increased functionality, and reduce power requirements.
Frequency selective components, that are used in oscillators, for example, are used in a wide variety of electronic circuits, including communications circuits, and microprocessors. Traditionally quartz resonators have been used as frequency selective elements in oscillator circuits (e.g. Colpitts Oscillator, Pierce Oscillator). Unfortunately quartz resonators are costly, bulky discrete components.
Recently attention has turned to the field of Microelectromechanical Systems (MEMS) for an alternative to quartz resonators.
In order to integrate a MEMS resonator with an electronic circuit, it is necessary that its design be compatible with the materials and process used in fabricating the electronic circuit. One established and widely used, set of materials and processes are those used to fabricate Complementary Metal Oxide Semiconductor (CMOS) integrated circuits. CMOS is particularly suited to making lower power consumption digital integrated circuits. CMOS integrated circuits are commonly fabricated in N or P type monocrystalline semiconductor wafers. In certain CMOS fabrication processes deep anisotropic etching is used to form capacitors. Such capacitors are used to store a charge in CMOS based memory. It would be desirable to have a MEMS fabrication process that is compatible with CMOS processes and materials.
Although MEMS devices are small compared to equivalent discrete devices, they are typically large compared to integrated circuit electrical devices (e.g. transistors). The area occupied by an integrated circuit is significant in determining its cost. This is because the area of an integrated circuit determines the number of semiconductor die""s bearing the circuit that can be made simultaneously on a single wafer, and the cost of semiconductor processes are determined on a per wafer basis. It is desirable to have MEMS resonators that occupy a relatively small amount of area on a surface of a semiconductor die on which they are fabricated.
Another issue to be addressed in the design of MEMS resonators, is the minimization of the dissipation of vibrational energy associated with the resonance of the resonator into the substrate (e.g. die) on which the resonator is fabricated. A high rate of vibrational energy dissipation would lower the Quality (Q) factor of the resonator, and broaden its frequency response. For most electronic circuit applications, for example for oscillator circuits, it is usually desirable to have a frequency selective component that exhibits a narrow band frequency response. Thus, it is desirable to have a MEMS resonator that does not efficiently radiate vibrational energy.
In as much as MEMS resonators comprises a sprung mass, one potential drawback is that external shocks (e.g. caused by dropping) will jar the MEMS resonator, and generate spurious transients in the signal (e.g., oscillator output) generated using the MEMS resonator. Thus, it is desirable to have a MEMS resonator that is less prone to cause spurious signal transients when jarred.